Setup ci/cd for coding standards

[dhruvang21]

Add CI pipeline with coding standards and PHPUnit test workflows

Summary

Implements the CI/CD pipeline proposed in #274.

• coding-standards.yml two jobs run on every PR and push to master:
• lint runs php -l across all PHP files to catch syntax errors (addresses Parse error in test.php (unclosed braces?) #271)
• phpcs runs PHP_CodeSniffer with WordPress Coding Standards and reports violations inline as PR annotations via cs2pr (addresses Multi PHP + Multi Environment + Check all commits #212)
• phpunit-tests.yml spins up a MySQL 8.0 service container and runs the full prepare → test → report → cleanup cycle across a PHP matrix (7.4, 8.0, 8.2) to simulate hosting environments
• composer.json adds squizlabs/php_codesniffer and wp-coding-standards/wpcs as dev dependencies with composer lint, composer lint:errors, and composer format scripts matching WordPress develop conventions
• dependabot.yml adds weekly Composer package update checks alongside the existing daily GitHub Actions checks
• .gitignore excludes .cache/ (PHPCS scan cache)

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Co-authored-by: dhruvang21 <dhruvang21@git.wordpress.org>
Co-authored-by: kittenkamala <amykamala@git.wordpress.org>

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[kittenkamala]

Thank you so much for doing this @dhruvang21! Will you do me a favor and separate the lint fix commit out into a separate PR so the two changes can be reviewed and merged independently?

ac66fde

ty!!

[dhruvang21]

@kittenkamala i have created the separate PR #326 to review the lint fixes